In the world of medical wire, Fort Wayne Metals is known for many things. Our high-strength 304V stainless steel wire is the workhorse material for many vascular interventional tools. Our DFT® composite wires combining Co-based 35N LT® and silver are extensively used in cardiac rhythm management and neurostimulation leads. Our superelastic Nitinol and Nitinol-composite wire is found in diverse applications ranging from orthodontic archwires to cerebrovascular flow diverters and dynamic bone nails where it improves patient well-being and saves lives. These materials have one thing in common: they are designed to be chemically stable in the body, remaining intact indefinitely. With an eye to the future, Fort Wayne Metals is working to develop materials capable of chemically dissolving over time, leaving behind only well-healed tissue.

With the recent approval of two magnesium-based implants in Europe, we may be at the beginning of a paradigm shift in medical intervention. Soon, devices which only need to provide support for a finite amount of time, such as stents, staples, and screws, may be predominantly made of absorbable metals which dissolve harmlessly and even beneficially over time. While some absorbable polymers are used today, they lack the strength and stiffness to be effectively deployed in many applications. Nutrient metals, composed primarily of elements already present in the body like magnesium, iron, or zinc, may be just the solution for these restorative therapies.

This concept, while revolutionary, is not new. Pure magnesium wire was used as a blood vessel ligature as early as 18781, and many reports of orthopedic fracture fixation with magnesium hardware can be found in the literature in the first decades of the 20th century2,3. However, largely due to a lack of materials performance controls, results were inconsistent at best. Once reliable stainless steels were made commercially available, the absorbable metal concept was abandoned. That is, until recently.

Of the three nutrient metal classes (Mg, Fe, Zn), magnesium has, of late, received the most attention and is promising for a wide array of medical applications.

In non-medical sectors, magnesium alloys are typically selected for engineering applications for one of three properties. First, magnesium has the lowest density of all structural metals (1.7 g/cm3), so it is commonly used to reduce component weight. Second, magnesium can burn when heated to its melting temperature (650°C) in the presence of oxygen, and decoy flares used to evade heat-seeking missiles exploit this behavior. Finally, magnesium is the most anodic of all structural metals, which makes it prone to corrosion. While in many instances corrosion is undesirable, it is sometimes used advantageously as a sacrificial anode to protect more noble metals, like steel.

It is exactly this tendency to corrode which makes magnesium a prime candidate for absorbable metals. It can be safely assumed that a magnesium component will dissolve by corrosion in the body (Figure 1). The challenge, then, is to make sure it does so at the right rate and in the right way. Strategies to tailor corrosion rates include alloying, thermo-mechanical processing, and coatings. Pure magnesium corrodes too quickly for most applications and is consequently alloyed with other elements to reduce the corrosion rate. Proper thermo-mechanical processing can greatly refine the microstructure and lead to a more uniform corrosion rate. Coatings can include absorbable polymers like PLLA or ceramic conversion coatings like MgF2 and various oxides. The reader is directed to the review by Zheng, Gu, and Witte for a more in depth discussion4.

Magnesium’s mechanical behavior can present some challenges to designers. Its strength, even when highly alloyed and processed, is relatively low when compared to materials like stainless steel or titanium (Table 1). This must be accounted for when designing the implants. The hexagonal-close-packed crystal structure of magnesium also produces modest ductility which must be accounted for in highly plastic applications (like balloon expandable stents or staples).

Table 1. Comparison of mechanical properties of magnesium alloy with common medical alloys.

Material

Mg Alloys

316LVM Stainless Steel

Ti-6Al-4V ELI

Ultimate Tensile Strength (MPa)

150 – 550

600 – 1750

1000 – 1500

Elongation (%)

2 – 25%

2.5 – 50%

3 – 15%

Young’s Modulus (GPa)

35-45

190

100

Even with these challenges, magnesium alloys are being successfully deployed in the clinic in both the orthopedic and cardiovascular space. Syntellix (Hannover, DE) was the first company to receive CE marking for a magnesium based implant, a compression screw with indications for bunion repair. It has been shown to be clinically equivalent to titanium screws for the same indication5 and is mostly absorbed 3 years post-implantation with substantial implant-site bone regrowth6. Biotronik (Berlin, DE) was recently awarded CE marking for their magnesium-based coronary stent. The BIOSOLVE-II clinical study of 123 patients has shown the magnesium was largely gone, with no definite or probable scaffold thrombosis, out to 127 and 24 months.

There are also efforts underway to begin producing standards for testing devices made of absorbable metals. Both ASTM and ISO are working jointly, with inputs from experts in industry and academia, to produce guidance documents for assessment areas including metallurgical characterization (ASTM F3160), in vitro degradation testing, corrosion fatigue testing, and biological evaluation. These documents, once complete, will serve to guide designers in the development and regulatory approval process.

Magnesium Wire

Seeing the benefit of medical implants made with magnesium, and given our deep expertise in wire processing, the R&D team at Fort Wayne Metals has been working to produce and tune wire in a variety of magnesium alloys. Prior to this work, magnesium wire was scarce to unavailable, especially in fine sizes required for medical devices. This was primarily due to the difficulty in working Mg at room temperature without cracking, and limited demand for such wire. We are leveraging our world-class wire drawing systems and our knowledge of the medical device market to overcome these challenges and meet burgeoning demand for the technology.

With our processing, we have successfully produced wire from many Mg alloys in diameters relevant for medical devices (bar as large as 10 mm, wire as small as 0.02 mm). These alloys have ranged from high ductility Mg-Li8 alloys to high strength Mg-Rare Earth alloys9,10, and alloys which only contain nutrient elements like Zn, Mn, and Ca. Each alloy will offer a different combination of strength, ductility, corrosion resistance, and biological response and choosing the right alloy for the application at hand is critical. In all cases, the patent landscape must be considered as well.

Figure 2. Magnesium alloy wire in both the cold-worked and annealed conditions is readily formable. Shown here is 0.3 mm cold-worked Mg-RE wire in various forms.

Resoloy®

One alloy which shows excellent promise is Resoloy®. Resoloy® was developed specifically for absorbable stents by MeKo (Sarstedt, Germany) and is patented in the US (US9522219 B2), Europe (EP2744532 B1), and China (CN103889475 B). We have devoted considerable attention to this material for its promising early results. A magnesium-rare earth alloy with Dysprosium as the main alloying element, it has excellent strength and ductility. The corrosion rate, like all magnesium alloys, depends on a host of factors, but is relatively low and uniform. Fort Wayne Metals has an arrangement with MeKo to provide Resoloy® in wire form to the medical device market, providing a key material with good freedom-to-operate.

Our cold-drawing techniques allow for very fine microstructures (Figure 3) and mechanical strengths which are uniquely high in the magnesium space. For example, cold worked Resoloy® wire can reach strengths exceeding 500 MPa, and after cold-working, heat treatment allows for very precise tuning of mechanical properties (Figure 4).

Figure 4. Tensile data for .127 mm Resoloy® wire produced with 70% cold work (As Drawn), and then heat treated at various temperatures.

In Closing

Healthy restoration of tissue, after injury or device implantation, can be enabled through absorbable magnesium technology, that “steps out of the way” once it is no longer needed. Living bone, and other tissues, are dynamic: they are constantly produced and absorbed to maintain healthy form and function11. While the strength levels of magnesium are less than typical metals, they are stiffer and stronger than most polymers, and comparable to or stronger than cortical bone. Future devices will incorporate absorbable magnesium wire that is well-engineered to the application, providing restorative solutions in vascular, orthopedic, and other regenerative therapies.

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